Communication networks need to keep up with the growth of today's Internet data traffic. The telecommunications industry needs new photonics equipment to improve current optical networks and for deployment in next generation optical networks. Semiconductor lasers are among the most important generation components in optical telecommunication systems. Optical linewidth of semiconductor lasers is important because linewidth determines the laser's coherence length and phase noise. The maximum data rate in an optical fiber communications link is determined by the ratio of signal power to noise power as per the Shannon-Hartley equation. Narrow linewidth is an essential requirement for lasers used in high data rate coherent communications, since phase noise impacts signal noise by the coherent detection process. Even non-coherent modulation schemes can suffer from a reduction in signal quality when phase noise is translated into amplitude noise. Thus, lasers for modern optical communications systems now require linewidths of a few hundred KHz or less.
Unfortunately semiconductor lasers typically have linewidths on the order of several to tens of MHz. Consequently, techniques for reducing optical linewidth of semiconductor lasers have been of growing importance since the move to coherent optical communications that has been building over the last decade.
Accordingly, there has been a significant amount of interest in optical coherent comb lasers (CCLs) and their benefits as a source of multiple spectral lines (also known as “tones”) in coherent optical fiber communications, because CCLs have been used to create the carrier frequencies in dense wavelength division multiplexing (DWDM) optical systems with net data rates exceeding Terabit/s transmission rates and high spectral efficiency [1-3]. Different techniques have been used to generate multi-wavelength lasers: modulator-based comb sources [3], spatial mode beating within a multimode fiber section [4], multi-cavity oscillation [5], comprising highly nonlinear fibers for spectral broadening [6], or high-Q microresonators [7]. However, these techniques either require complex setups with discrete components, high pump powers with delicate operating procedures, or they provide only a limited number of spectral carriers.
For practical systems, a compact, low-cost, energy-efficient CCL is desired. Applicant developed nanostructured InAs/InP quantum dot (QD) multi-band (multi-colour) multiwavelength mode locked laser, and has demonstrated intra-band and inter-band mode-locking (U.S. Pat. No. 7,991,023). Its use as a coherence comb laser (QD-CCL) over a large wavelength range covering C- or L-band has been demonstrated [8-18]. Unlike uniform semiconductor layers in most telecommunication lasers, in the QD CCL, light is emitted and amplified by millions of semiconductor QDs (typically less than 50 nm lateral diameter). Each QD acts like an isolated light source acting independently of its neighbours, and each QD emits light at its own respective wavelength. By providing high efficiency QDs with a desired emission frequency distribution, the CCL is more stable and has much better performance compared to other multi-wavelength lasers. Importantly, a single CCL has been shown to simultaneously produce 50 or more separate lines at spatially distributed wavelengths over the telecommunications C-band or L-band. To achieve these properties we have put considerable effort to design, grow and fabricate InAs/InP QD gain materials and produce CCLs.
More recently Applicant has demonstrated CCLs with repetition rates from 10 to 437 GHz and a total output power up to 50 mW, at room temperature [8-18]. Applicant has investigated relative intensity noises (RINs), phase noises, RF beating signals and other parameters of both filtered individual channels and the whole CCL's output [17-18]. Unfortunately, the single filtered channels of QD CCLs generally exhibit strong phase noise and broad optical linewidths, typically of the order of MHz [17-21]. As a consequence, wavelength-division multiplexing (WDM) data transmission using these CCLs has been restricted to direct detection schemes [22] or differential quadrature phase shift keying (DQPSK), which only uses relatively few (4) symbols. While these CCLs have high symbol rates, their aggregate data rates (up to 504 Gbit/s [23]) are limited by the symbol sets. Coherent transmission can use many more than 4 symbols to achieve higher baud rates, where linewidth allows. The CCLs are not satisfactory for Tbit/s (and higher) coherence optical networking systems.
Furthermore, other uses for CCLs, such as in high precision optical measurement devices or high resolution spectral analysis, are limited by this phase noise.
In order to improve net data transmission rates and spectral efficiency in optical coherent communication systems, researchers have put significant efforts to simultaneously reduce optical linewidth of each individual channel of CCLs. For example, a feed-forward heterodyne scheme has been used to simultaneously reduce the optical linewidth of many comb lines from mode-locked lasers [24-25]. Both [25], and [26] use a local oscillator (LO) and a Mach-Zender Modulator (MZM). The LOs have a narrow linewidth (narrower than the narrowest linewidth achieved by the feedforward system). These references show the difficulty of producing a large set of comb lines (more than 20) simultaneously narrowed to a high degree (below a few hundred kHz), even when resorting to the relatively complex setups.
Prior art for reducing linewidth of single mode lasers are also known. For example, U.S. Pat. No. 8,804,787 to Coleman et al. claims a particular arrangement for tapping a laser signal from a single mode laser cavity, attenuating the laser signal, and feeding the attenuated (−30 to −80 dB) laser signal back into the laser cavity, where the laser driver provides sufficient drive stability so that a frequency variation of the laser is less than a free spectral range (FSR) of the secondary cavity. This patent specifically identifies as an unexpected result: “that an uncontrolled OPL[Optical Path Length] to the back reflection provided by the first branch provides significant spectral narrowing, which can be several orders of magnitude narrowing”. A reduction of linewidth from 118 kHz to 2 kHz was demonstrated for a single wavelength QD laser. “Polarization Maintaining (PM) fiber or non-Polarization Maintaining SM fiber” can be used.
Recent papers [29,30] associated with a European Commission EC-FP7 Big Pipes project demonstrate simultaneous linewidth narrowing of 60 lines in a Quantum Dash mode-locked laser diode using resonant feedback from a secondary cavity, without any LO. The secondary cavity is provided with a freespace optical setup from a backside facet of the mode-locked laser diode that is barely disclosed. Freespace optical waveguides are typically polarization maintaining. Stability of the linewidth is not discussed in any of the prior art references, including these recent papers. Stability is particularly important for commercial deployment of lasers used in telecommunications applications. Given the highly schematic description of the optical system provided in these papers, it is unclear what kind of stability could be provided with their system. Given that “the external cavity length is adjusted to be near a multiple [M] of the optical length of the laser”[30], and a known variably of the laser optical length in operation, it is a safe assumption that the stability is poor outside of highly controlled lab settings. It should be noted that a large OPL for the external cavity (which would be desirable for a large linewidth reduction factor) will require this multiple M to be large. However if M is large, a small variation in the laser's effective OPL (S) will generate a difference MxS in the distance of the reflector from the intended position. The ability to predict or adapt the OPL of the external cavity is not trivial, if possible, and both the OPL of the external cavity and the attenuation have cumulative effects in terms of varying output, leading to a further source of instablility.
Accordingly there is a need for a technique for concurrently narrowing linewidths of a plurality of mode-locked comb lines in a CCL, without relying on a narrow linewidth LO and MZMs, without reducing a number of lines of the CCL, while retaining stability of the narrowed linewidth. Furthermore, there is a need for stably narrowing more linewidths of a CCL, to a greater extent, without complicated and expensive equipment to setup and maintain.